南大巴山前陆冲断带构造样式及变形机制分析

STRUCTURAL STYLE AND DEFORMATIONAL MECHANISM OF SOUTHERN DABASHAN FORELAND FOLD-THRUST BELT IN CENTRAL CHINA

大巴山构造带位于秦岭造山带和四川盆地的过渡部位,形成于印支-燕山期,定型于喜山期。按照构造变形样式及其组合特征,从北东向南西可依次划分为北大巴山逆冲推覆构造带、南大巴山前陆褶皱-冲断带(又包括叠瓦断层带、断层-褶皱带和滑脱褶皱带等3个亚带)和四川盆地东北部低缓构造区等3个构造带(区)。南大巴山冲断带地表构造以类侏罗山式褶皱为显著特征,主要发育叠瓦断层系、断层相关褶皱、被动顶板双重构造、反冲断层系和冲起构造等变形样式。北东-南西向挤压应力和滑脱层是控制南大巴山及其前缘构造变形的主要因素,结合区域地质研究成果,建立了南大巴山及其前缘地区依次从震旦系-下寒武统-志留系-中下三叠统逐渐抬高的多层次滑脱前展模式。

Based on field investigation, reflecting seismic profiles and other data, the Meso-Cenozoic structural style in southern Daba Mountains is discussed in this paper, and the deformation mechanism and tectonic model are outlined as well. The Daba Mountains lies between the Qinling orogenic belt and the Sichuan cratonic basin. Tectonically, from orogenic belt towards craton, the mountain ranges and their adjacent regions can be divided by Chengkou fault and Tiexi-Wuxi concealed fault into three tectono-stratigraphic units, namely, the northern Dabashan thrust-nappe belt, the southern Dabashan foreland fold-thrust belt, and the Sichuan basin. The southern Dabashan fold-thrust belt is a southwestward-bulging thin-skinned thrust wedge superimposed by dextral transpressive deformation, shows obvious zonation in deformation style and deformation intensity in the direction of dip, and can be divided further into two subzones, i. e. , the imbricate thrust zone characterized by imbricate stepped-thrust sheets, and the detachment-fold zone characterized by box fold and closed overturned-isoclinal fold on the outcrops. Several types of structures have been recognized or inferred, including imbricate thrust system, passive-roof duplex （triangle zone）, fault-related folds, back-thrust system and pop-up structure, and so on. NE-SW compressive stress from Qinling orogenic belt and detachment layers in the overlying strata are the two most important determinants of deformation style. After the collision between North China plate and Yangtze plate at the end of middle Triassic, the northward intracontinental subduction along the south edge of the Qinling orogenic belt was initiated, which led to the corresponding southward thrusting in the upper crust. At about the end of early Cretaceous, the thrusting was propagated southwestwards and extended to the southmost part of southern Daba Mountains, and caused thrusting deformation to be preferentially developed along major detachment layers and to progress toward foreland from lower Sinian through lower Cambrian and Silurian to middle and lower Triassic.